1. Technical Field
This invention relates to the technology of providing a wear resisting coating on aluminum or other light metal substrates, and more particularly to the provision of iron based coatings containing a self lubricating phase in the form of Fe.sub.x O.
2. Discussion of the Prior Art
To reduce weight and improve fuel efficiency, light weight aluminum block engines are being used more extensively throughout the automotive industry. Although aluminum block engines reduce weight, it is necessary to provide a more wear resistant cylinder bore surface for extended durability. Lightweight aluminum block engines incorporate either cast-in-place or pressed-in-place cast iron liners to provide a wear and scuff resistant cylinder bore surface. Use of cast iron liners for aluminum engine blocks has been known for some time (see U.S. Pat. No. 1,347,476). The functionality of such liners is based on compatibility between a steel piston ring pack in lubricated running contact with the cast iron cylinder bore wall. The tribological properties of grey cast iron make it an excellent material for cylinder bore applications providing the necessary wear and scuff resistance required to insure long-term durability and reliability. Metallurgically, the wear resistance and scuff resistance of grey cast iron can be attributed to the presence of graphite, a self lubricating phase which is uniformly distributed in a wear resistant matrix consisting of alpha-iron (Fe) and iron carbide (Fe.sub.3 C-cementite) phases. Although aluminum block engines currently incorporate cast iron liners, the cost and complexity associated with cast-in-place or pressed-in-place liner technology make alternative cylinder bore surfacing technology attractive.
Alternative surface technology heretofore has included nickel plating of cylinder bore walls to provide corrosion resistance to iron substrates while offering only limited reduction of friction because of the softness and inadequate formation of nickel oxide (see U.S. Pat. No. 991,404). Chromium or chromium oxide coatings have been selectively used in the 1980s to enhance wear resistant of engine surfaces, but such coatings are difficult to apply, are unstable, very costly, and fail to significantly reduce friction because of their inability to hold an oil film, have high hardness, and often are incompatible with steel piston ring materials. Aluminum bronze coatings have been applied to aluminum engine bores in the hopes of achieving compatibility with steel piston rings.
In the same time period, iron or molybdenum powders have also been applied to aluminum cylinder bore walls in very thin films to promote abrasion resistance. Such systems do not control the oxide form so as to possess a low enough coefficient of friction that would allow for appreciable gains in engine efficiency and fuel economy. For example, (as shown in U.S. Pat. No. 3,900,200) thermally (plasma) sprayed Fe.sub.3 O.sub.4 particles were deposited onto a cast iron substrate to obtain an increase in wear resistance (scuffing and abrasion resistance). Unfortunately, such coating eliminated the beneficial effect of a self lubricating phase. Similarly, in U.S. Pat. No. 3,935,797, an iron powder coating of 0.3% carbon was plasma sprayed onto an aluminum substrate propelled by a spray of inert gas resulting in an iron and iron oxide coating that inherently contained Fe.sub.3 O.sub.4 due to the excess of O.sub.2 drawn in by the spray action of the propellant. To decrease scuffing, a manganese phosphate coating was needed over the iron and oxide coating.